I run my DiabloMiner inside a tmux session (same as screen, but better).
I have to always export DISPLAY=:0.0 before launching a new session if I create a new tmux window.

There is a fairly complete guide about DiabloMiner here: http://forum.bitcoin.org/?topic=3356.msg47489#msg47489

If you have Crossfire, you should allegedly turn that *off*: http://forum.bitcoin.org/index.php?topic=7167.msg105782#msg105782

Check what the env var $COMPUTE is, as when that is set it supersedes $DISPLAY for ATI computation.

https://en.bitcoin.it/wiki/Mining_hardware_comparison only lists other miners for the 6990, not DiabloMiner.

Hope anything above helped...